(a) Field of the Invention
This invention relates to the gaseous treatment of molten metal using gas; more specifically the invention relates to a method and apparatus for the homogenization or degasification of steel or other metals.
(b) Description of Prior Art
With the advent of continuous casting of steel and with the demands for improved quality of steels, the use of inert gas for stirring molten steel is increasing. Inert gas is used to homogenize the molten steel in terms of its chemical analysis and temperature in the ladle after tapping from the refining furnace, and before teeming the steel into ingots or into continuous casting machines. By such homogenization a steel can be obtained the properties of which are more uniform throughout.
Several methods are known for the homogenization of molten steel. In one method homogenization is carried by agitating the molten steel by stirring it with a steel ingot, in a ladle, the ingot being lowered by an overhead crane into the molten steel in the ladle. The movement of the crane results in movement of the ingot and stirring of the metal. This method is cumbersome, time consuming and not thorough.
Methods are known which involve stirring with gas and these are more reliable. The methods of gas stirring differ in the way the gas is introduced.
One such method employing gas is the hollow stopper rod technique, in which a ladle stopper rod made of steel is used. The rod is hollow so that gas may flow through it; and the stopper rod head has small holes radially disposed to permit gas outflow; the rod is protected by refractory sleeves. In operation the rod assembly, rather than being fixed to the ladle, is attached to a fixed elevated horizontal beam, the rod is placed with the head downwards and gas flows into the stopper rod from a gas inlet, at the top and out at the bottom, through holes in the rod head. A ladle containing molten metal is brought by a crane to a location underneath the rod assembly, and is raised so that the assembly is immersed in the molten metal and gas bubbles from the stopper rod head through the molten metal, producing the desired agitation.
This method is used commercially; however, such a stopper rod assembly only lasts from 5 to 10 treatments, after which it is replaced. Also the gas flow into the molten metal is not reliable since the refractory sleeves make a poor seal against the steel stopper rod. In view of this, the gas takes the path of least resistance between the steel rod and the refractory sleeves rather than into the molten metal. Finally, consistent stirring can only be ensured by observing the agitation at the surface of the molten metal and adjusting the gas flow until the desired degree of agitation is observed.
In another known method a porous refractory is used to introduce gases into molten metal; this method is practiced commercially. The method utilizes a refractory brick or porous plug which has the property of being permeable to gas under pressure, but substantially impervious to molten metal. The porous plug is provided as part of the ladle lining at a location where it is submerged when the ladle is filled with molten metal. The gas is introduced into the molten metal through the porous plug, the desired degree of stirring being achieved by altering the gas flow rate into the porous plug. Depending on local conditions, such porous plugs can have a life expectancy of 10 to 25 cycles. With molten cast iron analysis, the life can be 50 to 200 heats.
Yet another method employs a metal tube technique; this method is used in some plants. A steel tube is immersed into the refractory lining of a ladle, for example at or near the bottom, leading through the outside steel shell and refractory lining and terminating on the inside surface of the refractory. The flow of gas is started before the metal is tapped into the ladle. After the desired agitation, the flow of gas is stopped and the metal flows back into the tube and freezes. As a rule, the tube must be replaced after each cycle. Sometimes the used tube can be cleared by prodding with a steel bar or by drilling out the frozen metal. The tube can be reused until it becomes too short.
A modification of the metal tube technique is employed in U.S. Pat. No. 3,395,910--Ronald L. W. Holmes--issued Aug. 6, 1968; in this modification a metal tube is used having a nozzle; the tube passes through the casing and refractory lining of the ladle and the tube and nozzle are encased with a sheath of low grade refractory cement which in turn is surrounded by a sheath of high grade refractory brick; the refractory cement is molded around the nozzles so as to provide a discharge passage extending axially from the discharge end of the nozzle. The overall sheath thus prevents contact between the molten metal and the metal tube; the tube further includes a highly conductive stopper rod member at its inlet so that molten metal freezes there without entering the gas supply lines, in the event of a sudden loss of gas pressure causing the molten metal to enter the metal tube. The refractory sheath is designed to be replaced after every heat.
The porous refractory and metallic tube techniques to introduce a relatively non-reactive gas into the bottom of a molten metal bath is being practiced in some LD oxygen steelmaking converters to equilibrate the molten metal and slag.
In electric furnaces reverberatory furnaces and the like, these gas dispensing devices can also be used to stir the molten metal.